The invention relates to the mounting of electronic components. More specifically, it relates especially to the mounting of surface packages having matrix-type peripheral outputs of the BGA (ball grid array) or CSP (chip-scale package) type on printed circuits, and also of the FCOB (flip chip on board) type, as well as the assembly of these packages themselves.
These packages contain a silicon chip—an integrated circuit—and are intended to be mounted on cards which are then fitted into electronic appliances. At the present time their preferred mass application is in advance-technology GSM portable telephones. Whereas in previous-generation packages the connections to the printed circuit are provided by pads located around the periphery of a package, for example of the QFP (quad flat package) type or the like, BGA, micro-BGA or CSP packages include, for this purpose, contact elements in the form of balls of an Sn/Pb eutectic alloy containing 63% tin and 37% lead (183° C. eutectic alloy; optionally, a few per cent of silver may be added thereto). The package is assembled by reflow soldering of the balls to the printed circuit at the same time as the other components of the card.
Multilayer printed circuits usually consist of a stack of layers each having copper conducting tracks connected by interconnection vias and deposited on an insulating substrate, for example made of epoxy resin/glass fibre. The printed circuit also includes, on its external sides, copper pads for receiving the component to be soldered, each receiving pad coming into contact with a ball of the package when the latter is positioned by a positioning machine. The copper receiving pad has been coated by the circuit manufacturer with a top coat comprising a nickel sublayer 5 to 10 μm in thickness and a gold superficial film 500–1000 Å in thickness for oxidation resistance, forming a plane and solderable top surface.
It should be noted that nickel, if it has been deposited by a chemical (and therefore electroless) process contains up to 10 at % of phosphorus since this element is associated with the formulation of the nickel solution used for chemically depositing the nickel. The package is joined to the printed circuit by soldering in a reflow oven. For this purpose a spot of solder cream is deposited on the printed-circuit regions to be soldered, the spot of solder cream comprising microballs of a tin/lead alloy of the same composition as the balls of the package, a solder flux and also various organic additives, activators and texture agents. After screen-printing the solder cream and then placing the package on the printed circuit, the card is made to pass through the oven on a conveyor, in order to carry out, in a single pass, the soldering therein, according to a precise temperature/time profile, in a nitrogen atmosphere or in air. The solder cream and the alloy balls are heated in several steps with heating rates typically of about 1.2 or 3° C./s up to 220/230° C., i.e. a temperature above the eutectic temperature. The time spent in the liquid state typically varies in total between 40 s and 90 s. The maximum temperature is the reflow peak after which the cooling starts. The cooling is usually carried out at a rate which does not exceed 2° C./s, and generally is from 1 to 2° C./s. It is possible, for example, to remove hot nitrogen present in the oven and make it flow through a “cold box” which also serves to trap the vapours coming from the solder flux and is cooled by circulation of water at 10° C. This cooled nitrogen purified of flux is reinjected into the oven, at the point where the reflow peak is reached, in order to cool the card.
The soldered joint, which has therefore been produced after the soldering operation, forms a mechanical and electrical connection which is stable and durable and comprises several successive layers:
the copper of the receiving pad of the printed circuit;
a nickel layer containing phosphorus if this nickel is of chemical origin;
a layer of a tin/nickel/phosphorus intermetallic compound (having a thickness of the order of 1 μm); and
the 63/37 tin/lead alloy.
Since the gold is dissolved by the tin/lead alloy during the soldering, it no longer plays any role in the process.
It should also be noted that the implantation of the tin/lead alloy balls in the BGA package itself is carried out in a very similar sequence of operations, and involving the same metallurgical operations. The bottom of the resin package has copper pads treated with a nickel/gold top coat, on each of which is placed an adhesive flux and then a ball of tin/lead alloy. The whole assembly is then put into an oven in order to reflow-solder the balls to the receiving pads.
The presence of the layer of intermetallic compound causes a problem when this is a nickel/tin/phosphorus ternary mixture as this compound exhibits brittle-type mechanical behaviour. This intrinsic brittleness makes the package/printed-circuit bond or the ball/package bond impact-sensitive, the more so as the actual geometry of the ball/chip contact region results in a stress concentration in this same contact region. A mode of fracture called brittle fracture, associated with the rapid propagation of microcracks, is therefore particularly liable to occur therein when the package or appliance into which the card is integrated is subjected to an impact. This phenomenon occurs particularly during the testing, which consists in dropping an appliance (such as a portable telephone) several times from a predetermined height (the test usually being called a “drop test”).
In the case of card/package bonds, it is customary to try to remedy this problem by reinforcing the mechanical bond by injecting resin under the package at the base of the balls after soldering (called “under-filling”). This resin also has the advantage of absorbing the mechanical stresses associated with the differences in thermal expansion coefficient between the various materials of the system. However, this application requires an additional fabrication step, which complicates the fabrication of the card and increases its manufacturing cost.